This invention relates to a hermetic sealing cover for a container and, more particularly, to a sealing cover adapted to seal a semiconductor device in the cavity of a body in which the cover facing the bonding wires of the semiconductor device is insulated so that bonding wires accidentally touching the cover, if metal, will not cause an electrical short thereby making the semiconductor device inoperable.
There are described and claimed in U.S. Pat. Nos. 3,823,468 and 3,874,549 a metallic sealing cover and preattached metallic solder preform which is then applied to cover the cavity in a metallic or ceramic container for a semiconductor device and is attached thereto by heating the device to the fusing point of the solder, thereby permanently hermetically sealing the container. Such covers are entirely satisfactory but are of relatively high cost.
In the semiconductor industry there have come into use various plastic preforms as a substitute for the metallic solder preform for sealing the cover to the semiconductor container. These plastic preforms have been used in sealing covers and containers, each of which may be of metal, ceramic, quartz, or plastic and used in various combinations. The principal advantage of the use of the plastic preform is low cost. It also has the advantage of sealing some materials to each other which cannot be accomplished with a metallic solder. Conventionally, the plastic preform is placed in registry with the cavity opening and the cover is placed in registry with the assembly and sealing is accomplished by the application of heat and pressure. To a small extent, the cavity openings have been sealed by the use of a plastic preform and cover assembly in which the plastic preform has been preattached to the cover by applying heat.
When using plastic sealing performs as described with a cover which is an electrically conductive metal, the area of the cover surrounded by the plastic preform and which faces the inside of the cavity after sealing is exposed. Should any of the connecting wires which are attached to the active semiconductor device come in contact with the exposed metallic surface of the cover, an electrical short may result which renders the semiconductor device nonfunctonal and useless.
Preattaching the plastic preform to the cover by heat, as described, gives rise to a second disadvantage, namely, the degrading effect the application of heat has on the ability of certain types of plastic preforms effectively to form a seal to the surface surrounding the cavity in the semiconductor container. Such plastic preforms are necessarily of thermosetting plastic material in which a chemical reaction takes place when heat is first applied such that the material becomes irreversibly at least partially hardened. If the thermosetting plastic is not in contact with the surface to which it is to be sealed when heating takes place, the exposed surface hardens and loses its adhesive quality. When a thermosetting plastic preform such as epoxy is preattached to the cover by applying heat, adhesion is obtained between the cover and the surface of the preform in contact with the cover. However, the other surface of the preform also becomes at least partially hardened and loses some or all of its adhesive qualities. When the cover-preform unit is placed in registry on the semiconductor package cavity and heat is applied for final sealing, a degraded seal is obtained between the semiconductor container and the thermosetting plastic preform. As a result, many such seals leak, thereby nullifying the desired goal of forming a durable hermetic barrier between the active semiconductor element in the container and the ambient atmosphere. Often, the adhesive quality of such a preattached plastic preform is so degraded that the cover with its preform falls off the package with the slightest mechanical distrubance.
It is an object of the present invention, therefore, to provide a hermetic sealing cover and preattached plastic sealing preform which completely eliminates one or both of the above-described disadvantages of prior plastic sealing cover-preform units of the type described.
It is another object of the invention to provide a new and improved hermetic sealing cover and preattached plastic sealing preform which is of low cost and which provides a durable hermetic seal for a semiconductor package.